The present invention relates to a method for producing optical media discs.
Optical media discs have been prepared previously using molding techniques such as those described in The Compact Disc Handbook, 2nd edition, by Pohlmann. Methods of molding optical media discs are well known in the art and include injection and injection-compression molding. Compact discs (CD's) have a thickness of approximately 1.2 mm and are typically made using standard injection molding techniques. However, Digital Versatile Discs (DVD's) have a layer thickness of 0.6 mm and require injection compression-molding as described in Injection Molding An Introduction, pgs. 171-172 Hanser/Gardner/ Publication, Inc., Cincinnati, 1995 by Potsch and Michaeli. Injection-compression is an enhanced injection molding process where the mold is left slightly ajar at the beginning of the cavity fill stage. The cavity is clamped completely closed to the desired final part thickness of 0.6 mm during the filling phase. Injection-compression is required in order to achieve adequate data transfer to a 0.6 mm substrate surface while achieving low birefringence in polycarbonate discs. Unfortunately compression-injection molding requires increased processing complexity, thus requiring purchase of new processing equipment or costly modification of existing standard injection molding equipment.
Therefore, there remains a need for a process for producing DVD's using standard injection molding equipment while maintaining acceptable data transfer, cycle time and low birefringence.